533 research outputs found
Superconductivity in spinel oxide LiTi2O4 epitaxial thin films
LiTi2O4 is a unique material in that it is the only known oxide spinel
superconductor. Although bulk studies have demonstrated that superconductivity
can be generally described by the Bardeen-Cooper-Schreiffer theory, the
microscopic mechanisms of superconductivity are not yet resolved fully. The
sensitivity of the superconducting properties to various defects of the spinel
crystal structure provides insight into such mechanisms. Epitaxial films of
LiTi2O4 on single crystalline substrates of MgAl2O4, MgO, and SrTiO3 provide
model systems to systematically explore the effects of lattice strain and
microstructural disorder. Lattice strain that affects bandwidth gives rise to
limited variations in the superconducting and normal state properties.
Microstructural disorder such as antiphase boundaries that give rise to Ti
network disorder can reduce the critical temperature, but Ti network disorder
combined with Mg interdiffusion can affect the superconducting state much more
dramatically. Thickness dependent transport studies indicate a
superconductor-insulator transition as a function of film thickness regardless
of lattice strain and microstructure. In addition, surface sensitive X-ray
absorption spectroscopy has identified Ti to retain site symmetry and average
valence of the bulk material regardless of film thickness.Comment: 25 pages, 7 figures, v2 - expanded Fig 1,2,7 with added discussion
Uncompensated magnetization and exchange-bias field in LaSrMnO/YMnO bilayers: The influence of the ferromagnetic layer
We studied the magnetic behavior of bilayers of multiferroic and nominally
antiferromagnetic o-YMnO (375~nm thick) and ferromagnetic
LaSrMnO and LaCaMnO (nm), in particular the vertical magnetization shift and exchange
bias field for different thickness and magnetic dilution of the
ferromagnetic layer at different temperatures and cooling fields. We have found
very large shifts equivalent to up to 100\% of the saturation value of
the o-YMO layer alone. The overall behavior indicates that the properties of
the ferromagnetic layer contribute substantially to the shift and that
this does not correlate straightforwardly with the measured exchange bias field
.Comment: 10 figures, 8 page
The role of magnetic anisotropy in spin filter junctions
We have fabricated oxide based spin filter junctions in which we demonstrate
that magnetic anisotropy can be used to tune the transport behavior of spin
filter junctions. Until recently, spin filters have been largely comprised of
polycrystalline materials where the spin filter barrier layer and one of the
electrodes are ferromagnetic. These spin filter junctions have relied on the
weak magnetic coupling between one ferromagnetic electrode and a barrier layer
or the insertion of a nonmagnetic insulating layer in between the spin filter
barrier and electrode. We have demonstrated spin filtering behavior in
La0.7Sr0.3MnO3/chromite/Fe3O4 junctions without nonmagnetic spacer layers where
the interface anisotropy plays a significant role in determining transport
behavior. Detailed studies of chemical and magnetic structure at the interfaces
indicate that abrupt changes in magnetic anisotropy across the
non-isostructural interface is the cause of the significant suppression of
junction magnetoresistance in junctions with MnCr2O4 barrier layers.Comment: 7 pages, 7 figure
Influence of chemical and magnetic interface properties of Co-Fe-B / MgO / Co-Fe-B tunnel junctions on the annealing temperature dependence of the magnetoresistance
The knowledge of chemical and magnetic conditions at the Co40Fe40B20 / MgO
interface is important to interpret the strong annealing temperature dependence
of tunnel magnetoresistance of Co-Fe-B / MgO / Co-Fe-B magnetic tunnel
junctions, which increases with annealing temperature from 20% after annealing
at 200C up to a maximum value of 112% after annealing at 350C. While the well
defined nearest neighbor ordering indicating crystallinity of the MgO barrier
does not change by the annealing, a small amount of interfacial Fe-O at the
lower Co-Fe-B / MgO interface is found in the as grown samples, which is
completely reduced after annealing at 275C. This is accompanied by a
simultaneous increase of the Fe magnetic moment and the tunnel
magnetoresistance. However, the TMR of the MgO based junctions increases
further for higher annealing temperature which can not be caused by Fe-O
reduction. The occurrence of an x-ray absorption near-edge structure above the
Fe and Co L-edges after annealing at 350C indicates the recrystallization of
the Co-Fe-B electrode. This is prerequisite for coherent tunneling and has been
suggested to be responsible for the further increase of the TMR above 275C.
Simultaneously, the B concentration in the Co-Fe-B decreases with increasing
annealing temperature, at least some of the B diffuses towards or into the MgO
barrier and forms a B2O3 oxide
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Phase transitions and magnetic domain coexistence in Nd0.5Sr0.5MnO3 thin films
We present a study of the physical properties of perovskite oxide Nd0.5Sr0.5MnO3 (NSMO) thin films grown on (110)-oriented SrTiO3 substrates. In bulk form, NSMO displays coupled magnetic and electronic transitions from paramagnetic/insulator to ferromagnetic (FM)/metal and then to antiferromagnetic (AFM)/charge-ordered insulator with decreasing temperature. In thin films, the AFM ordering only occurs when the films exist in an anisotropic strain state such as those obtained on (110)-oriented cubic substrates. In this work, resonant X-ray reflectivity, soft X-ray photoemission electron microscopy (X-PEEM), and magnetometry measurements showed that the NSMO film displays both vertical and lateral magnetic phase separation. Specifically, the film consists of three layers with different density and magnetic properties. The FM and AFM properties of the main NSMO layer were probed as a function of temperature using soft X-ray magnetic spectroscopy, and the coexistence of lateral FM and AFM domains was demonstrated at 110 K using X-PEEM
The role of hydrogen in room-temperature ferromagnetism at graphite surfaces
We present a x-ray dichroism study of graphite surfaces that addresses the
origin and magnitude of ferromagnetism in metal-free carbon. We find that, in
addition to carbon states, also hydrogen-mediated electronic states
exhibit a net spin polarization with significant magnetic remanence at room
temperature. The observed magnetism is restricted to the top 10 nm of
the irradiated sample where the actual magnetization reaches emu/g
at room temperature. We prove that the ferromagnetism found in metal-free
untreated graphite is intrinsic and has a similar origin as the one found in
proton bombarded graphite.Comment: 10 pages, 5 figures, 1 table, submitted to New Journal of Physic
Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by control of growth kinetics
We report on the selective fabrication of high-quality SrIrO and
SrIrO epitaxial thin films from a single polycrystalline SrIrO
target by pulsed laser deposition. Using a combination of X-ray diffraction and
photoemission spectroscopy characterizations, we discover that within a
relatively narrow range of substrate temperature, the oxygen partial pressure
plays a critical role in the cation stoichiometric ratio of the films, and
triggers the stabilization of different Ruddlesden-Popper (RP) phases. Resonant
X-ray absorption spectroscopy measurements taken at the Ir -edge and the O
-edge demonstrate the presence of strong spin-orbit coupling, and reveal the
electronic and orbital structures of both compounds. These results suggest that
in addition to the conventional thermodynamics consideration, higher members of
the SrIrO series can possibly be achieved by kinetic
control away from the thermodynamic limit. These findings offer a new approach
to the synthesis of ultra-thin films of the RP series of iridates and can be
extended to other complex oxides with layered structure.Comment: 7 pages, 6 figure
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